Positivity that isn't based on logic is wrong, just like negativity that isn't based on logic is wrong.
It's better to be honest with yourself about any given situation. Accept what you could have done better at without feeling inadequate. Recognize what you did well at without feeling complacent. You grow in response to your failures, not your successes.
Recognize what was good about the job and what you'll miss, so you can factor it into your next search. And obviously avoid the bad parts. If making yourself believe the job was only bad in an effort to save face and stay positive leads you to seek out the opposite, that's likely counter-productive.
These situations are rarely black and white. Everyone has a combination of failures/micro-failures, successes/micro-successes at every job whether they get fired/laid off or not. So there's plenty of logic to refraining from beating yourself up. Seeing a situation clearly and learning lessons from it happens more easily in hindsight than when you're stuck in it. So there's a lot of value in thinking about the past, if it's for the right reasons.
Staying dispassionate is more important than staying positive.